If it’s the right dream for you and it allows you to support yourself, so that you’re not pursuing your dream at someone else’s expense, it will probably last a lifetime. If it stops being enjoyable and fulfilling and you fall out of love with it—if it starts feeling like drudgery to the point where it becomes painful and depressing—there’s nothing wrong with saying, “Okay, what else do I want to do with my life?” and start pursuing that dream.